Obesity can be generally predictive of morbidities such as coronary artery disease and diabetes, and the anatomical distribution of adipose tissue (fat) can be a strong independent predictor of these and other medical conditions and outcomes. For example, overweight subjects with a larger proportion of fat stored as visceral adipose tissue (VAT) are believed to be at a higher risk than similarly overweight individuals with a larger percentage of fat stored as subcutaneous adipose tissue (SAT). Studies have shown that VAT levels are a predictor of cardiovascular risk factors, e.g. HDL, LDL, triglyceride levels, and hypertension. Because of the predictive and other values of visceral fat as distinguished from general obesity and subcutaneous fat, it is believed desirable to find a way to efficiently and effectively measure or estimate VAT.
It is known in the art to measure or estimate VAT by differentiating it from SAT in abdominal cross-sections or slices using computerized tomography (CT) and magnetic resonance imaging (MRI). Measurements can be made at the level of the umbilicus, where SAT and VAT volumes typically are identified by an image thresholding algorithm. However, the relatively high cost of both examinations and the high radiation dosage of CT can discourage the use of these techniques as a screening tool for VAT levels. Further, the thresholding method lacks specificity because areas or volumes above the threshold can have different amounts of % fat, and areas or volumes below the threshold may not be fat-free. Thus, systematic errors can be introduced by assumptions of % fat in areas or volumes above or below the threshold.
Dual-energy x-ray absorptiometry (DXA) exams are widely available, rapid, relatively low dose, and much less costly than CT and MRI exams. Further, DXA is capable of measuring both global and regional fat mass because, for tissue paths that are projected as pixels in the x-ray image, a given dual-energy x-ray measurements pertains to a unique combination of fat and lean mass. However, the ability of DXA to distinguish between VAT and SAT has been limited because DXA is a two-dimensional projection technique.